1. Field of the Invention
The present invention relates to a motor having a pulse mode and a brushless mode, and a motor control method or apparatus configured to control driving of the motor, and an optical apparatus using the motor control unit.
2. Description of the Related Art
Digital positional control over a stepping motor can be easily provided using an open loop control. Therefore, the stepping motor is widely used for an optical apparatus such as a camera, an exchange lens, an optical disk drive, a printer, and a projector.
The stepping motor may step out when the load to the stepping motor increases and the stepping motor is rotated at a high speed. One proposed, step-out preventive method attaches an encoder configured to detect a rotor's rotating position to the stepping motor, and switches electrification states to coils according to the rotor's rotating position so as to make the stepping motor act like a so-called brushless DC motor. In the following description, a driving mode of the stepping motor using the open loop control will be referred to as a pulse mode, and a driving mode using the control similar to the brushless DC motor will be referred to as a brushless mode.
Each of Japanese Patent Publication No. 06-067259 and Japanese Patent Laid-Open No. 2002-359997 discloses a motor control unit configured to advance according to the velocity a phase of a signal obtained by a noncontact sensor, such as a Hall device in the motor and to switch the current supplied to each coil according to the signal. The motor control unit solves a delay of leading of the current, and realizes high-speed rotation of the motor. At this time, a phase of the current (pulse) which is advanced in accordance with the velocity is referred to as a fast angle.
The motor control unit disclosed in Japanese Patent Publication No. 06-067259 utilizes a Hall device configured to output a two-phase signal that shifts by a phase of an electric angle of 90°. In the circuit configured to output a signal according to the rotating velocity of the rotor, a fast signal is digitally generated for each predetermined phase angle from the output signal of the Hall device, and the signal is selected and output so as to advance the phase.
The motor control circuit disclosed in Japanese Patent Laid-Open No. 2002-359997 measures a phase difference timing of the output signal of the Hall device using a timer, calculates a fast angle timing suitable for a fast angle amount stored in a memory based on the measurement value of the timer, and generates the fast signal so as to advance the phase. Each fast angle amount is set so that it can correspond to a velocity region of a rotation of the rotor, or so that a fast angle amount is separately set to a forward rotating direction and a backward rotating direction of the rotor.
Each of the motor control units disclosed in Japanese Patent Publication No. 06-067259 and Japanese Patent Laid-Open No. 2002-359997 generates a fast signal from the output signal of the Hall device by setting a preset fast angle. However, according to this method, lots of fast angle values to be prepared are necessary for finer positioning controls.
Alternatively, a delay angle that is a phase difference between the output signal of the Hall device and a delay drive pulse signal for driving the motor for the output signal of the Hall device may be varied (increased or decreased) by increasing or decreasing the delay angle by a predetermined value whenever the polarity of the drive pulse signal changes. In other words, a delay angle operation for advancing the phase by micro-changing the delay angle may be performed whenever the drive pulse signal is generated.
However, as the delay angle operation proceeds in the acceleration or deceleration control, a phase relationship turns between the drive pulse signal for driving the motor and the output single of the Hall device. In this case, the motor becomes out of control and the motor cannot be accelerated to a target maximum velocity because a drive pulse signal is generated based on the output timing of the reference output signal of the Hall device.
In addition, when the pulse mode is transferred to the brushless mode, it is necessary to select the reference output signal of the Hall device used to generate the pulsed signal for the drive pulse signal to be generated. In this case, when the output signal of the Hall device distant with respect to the signal output timing from the drive pulse signal to be generated is selected, a large delay angle can be obtained due to the phase difference from its position. Thereby, turning of the phase relationship between the drive pulse signal and the output signal of the Hall device may be delayed, or the motor becomes likely to be accelerated to the target maximum velocity without causing a reversal of the phase relationship even when the output signal of the Hall device is not switched.
Nevertheless, the delay angle obtained in transferring the pulse mode to the brushless motor contains a detection error, the influence of the detection error increases as the delay angle operation continues longer. Thereby, a phase difference shifts from the electric angle of 90° between the two-phase drive pulse signal (A-phase drive pulse signal and B-phase drive pulse signal).